The present invention relates to computer network jitter buffers and, more particularly, to jitter buffers that discard sample data or replicate sample data when the jitter buffers contain too much or too little sample data, respectively.
Packet-switched networks (such as local area networks (LANs) or the Internet) can be used to carry audio, video or other continuous signals, such as Internet telephony or video conferencing signals. In such an application, a sender and a receiver typically communicate with each other according to a protocol, such as the Real-time Transport Protocol (RTP), which is described in RFC 3550. The sender digitizes the continuous input signal, such as by sampling the signal at fixed or variable intervals. The sender sends a series of packets over the network to the receiver. Each packet contains data representing one or more discrete signal samples. (Sometimes, data representing a segment, such as a 10 millisecond segment, of the signal is referred to as a “sample,” even though such a sample includes many discrete digitized values. Discrete digitized values are referred to herein as “samples” or “sample data units,” which can be 8-bit bytes or other size data units.) The sender typically sends the packets at regular time intervals. The receiver reconstructs the continuous signal from the received samples and typically outputs the reconstructed signal, such as through a speaker or on a screen of a computer.
Optionally, the sender uses a compressor-decompressor (codec) to compress (also commonly referred to as “code”) the samples before sending the packets to the receiver. If the sender uses a codec, the receiver uses a compatible codec to decompress (decode) the samples before reconstructing the signal.
Senders and receivers use clocks to govern the rates at which they process data, however these clocks are typically not synchronized and typically operate at different speeds. This difference can cause a sender to send packets too frequently or not frequently enough, from a receiver's point of view, thereby causing the receiver's buffer to overflow or underflow. Furthermore, the Internet and most other networks, over which such real-time packets are sent, introduce variable and unpredictable propagation delays, which cause the packets to arrive at the receiver with variable and unpredictable inter-arrival times. This phenomenon is commonly referred to as “jitter.”
A jitter buffer is commonly used to compensate for differences in clock speeds between transmitters and receivers and variations in inter-arrival times of packets. A jitter buffer is an elastic store that accepts received packets whenever they arrive. Once the jitter buffer contains several packets, it begins supplying the packets to the receiver at a fixed rate. Generally, the elasticity of the jitter buffer enables the buffer to continue supplying packets to the receiver at the fixed rate, even if the packets from the sender arrive at the jitter buffer at a variable rate or no packets arrive for a short period of time.
However, if no or insufficient packets arrive at the jitter buffer for an extended period of time (as can occur if, for example, the network becomes congested), the buffer can become empty (“underflow”). An empty jitter buffer can not provide packets to the receiver, which causes an undesirable gap in the otherwise continuous signal output by the receiver until another packet arrives from the receiver. Such a gap is manifested as silence in an audio signal or as a blank or “frozen” screen in a video signal.
On the other hand, if more packets arrive at the jitter buffer over a short period of time than the buffer can accommodate (as can occur if, for example, a congested network suddenly becomes less busy), the jitter buffer can “overflow” and discard some of the arriving packets. This causes a loss of one or more entire packets of samples, which can cause an undesirable discontinuity or “jump” in the otherwise continuous signal output by the receiver.
A so-called “adaptive” jitter buffer can expand and contract (within limits), depending on the arrival rate of the packets. Although an adaptive jitter buffer is less likely to overflow than a fixed-size jitter buffer, an adaptive jitter buffer can experience underflow and cause the above-described gaps in the signal output by the receiver.